Used to spawn Hyoscya in Yorcia Weald at the northeast corner of F-8 (careful of chigoes), just north of Bivouac #3.
Oddly enough, the Corpse Flowers that drop these are nowhere near the ???. I scoured the map and here are where I found them:
Two at southern border of H-9
One at eastern border of G-9
One at eastern border of I-9
Three at eastern border of J-7
Two at northwest corner of J-7
One at northeast corner of I-7
One at southern border of I-7
Two where I-6 meets J-6
Two at the southern border of J-6
Five where J-6 meets K-6
Basically, your best bet for finding this is to take the Frontier Station waypoint and go to the northeast section of the map.
